This invention relates to image processing and more particularly to encoding of images for the efficient transmission and/or storage of high quality image information.
The demand for electronic services related to pictorial images or other two-dimensional data has grown so rapidly that even the accelerating advance of electronic transmission and storage technologies will not be able to keep pace, unless the electronic data derived from the images can be compressed in a way that does not impair perception of the reconstructed image or other two-dimensional data.
Different compression methods have evolved in the art as understanding of pictorial data has increased and theoretical advances have been made. Differential Pulse Code Modulation (DPCM) and bit-plane coding were among the early methods used, and they achieved compression factors of up to 4-6 by trading image quality for lower bit rate. Pictures with higher quality than obtainable with DPCM, coded with only one bit per pixel, can now be obtained with a number of methods, such as the Adaptive Discrete Cosine Transform (ADCT) described by W. H. Chen and C. H. Smith, in "Adaptive Coding of Monochrome and Color Images", IEEE Trans. Comm., Vol. COM-25, pp. 1285-1292, November 1987. In an ADCT coding system, the image is decomposed into blocks, generally eight by eight, and for each of the blocks a DCT (Discrete Cosine Transform) is carded out. The compression is obtained by quantization of the DCT coefficients with variable thresholds, partially optimized for the human visual acumen, followed by variable word length encoding.
Sub-band coding of images has been introduced to picture coding. One arrangement was proposed by J. W. Woods and S. D. O'Neil, in "Sub-Band Coding of Images", IEEE ASSP, Vol. 34 No. 5, October 1986, pp. 1278-1288. The arrangement proposed by Woods et al includes a filter bank, that divides the image signal into bands of different frequency content, and the signal of each filter output is compressed via DPCM. The compressed signals are then transmitted to a receiver where the process is reversed. Specifically, each signal is DPCM decoded and then up-sampled, filtered, and combined with the other filtered signals to recover the original image.
H. Gharavi and A. Tabatabai in "Sub-Band Coding of Images Using Two-Dimensional Quadrature Mirror Filtering," Proc. SPIE, Vol. 707, pp. 51-61, September 1986, use long complex quadrature mirror filters to obtain a number of frequency band signals. The "low-low" band is DPCM coded using a two-dimensional DPCM codec. A dead-zone quantizer is used for the other bands, followed by PCM coding.
Other sub-band coding schemes such as proposed by P. H. Westerink, J. W. Woods and D. E. Boekee in Proc. of Seventh Benelux Information Theory Symposium, pp. 143-150, 1986, apply vector-quantization techniques to code the filter bank outputs.
In the copending patent application of H. Bheda and A. Ligtenberg, Ser. No. 222,987, filed Jul. 22, 1988, and assigned to the assignee hereof, the data redundancies in the different sub-band signals are employed to achieve additional data compression. In fact, that technique provides an excellent "front end" for image processing based on sub-band analysis techniques.
There remains the problem of quantizing the analyzed information more effectively in terms of bits per pixel and perceived quality of a reconstructed image. We have determined that the existing versions of the Discrete Cosine Transform do not take full advantage of all facets of the known properties of human visual perception.
Some recent work has addressed this problem. See the article by King N. Ngan et al, "Cosine Transform Coding Incorporating Human Visual System Model," SPIE Vol. 707, Visual Communications and Image Processing (1986), pp. 165-171, particularly addressing contrast sensitivity. The contrast sensitivity is applied to the quantization process in a very restricted fashion; and other relevant parameters are not applied. Indeed, a kind of pre-emphasis is applied before quantization, apparently in preference to a more precise degree of control over the quantization process.